The invention relates to a system and a method for determining a characteristic of a blood vessel portion, which comprises blood including a contrast agent exhibiting resonant absorption of x-ray photons at a specific energy. The system comprises a tunable monochromatic x-ray source (21) emitting x-ray radiation, an x-ray detector device (22) for detecting the x-ray radiation after it has travelled through the blood vessel portion. A control unit (26) varies a tuning of the x-ray source (21) to vary the energy of the x-ray radiation emitted by the x-ray source (21), and an evaluation unit (27) determines a tuning of the x-ray source (21) at which nuclear resonant absorption of the x-ray radiation incident onto the blood vessel portion occurs and estimates the characteristic on the basis of the determined tuning. The characteristic may particularly be the blood velocity in the blood vessel portion.